Plating weight control device and control method

Abstract

An apparatus for controlling coating weight of a steel sheet by using an air knife includes: an air knife condition derivation unit configured to derive a first air knife gap and a final air knife pressure for target coating weight, and derive a second air knife gap for achieving the target coating weight at a current air knife pressure; and an air knife gap compensation unit configured to determine a final air knife gap according to a gap compensation amount that is a difference between the second air knife gap and the first air knife gap and a gap compensation ratio based on an air knife pressure variation amount for a control period, in which the control period is a period for updating an air knife condition for the target coating weight.

The invention claimed is: 
     
       1. An apparatus for controlling coating weight of a steel sheet by using two air knives, the apparatus comprising:
 an air knife condition derivation unit configured to derive a first air knife gap and a final air knife pressure for target coating weight, and derive a second air knife gap for achieving the target coating weight at a current air knife pressure; 
 an air knife pressure response compensation unit configured to determine a final air knife gap according to a gap compensation amount that is a difference between the second air knife gap and the first air knife gap and a gap compensation ratio based on an air knife pressure variation amount for a control period; and 
 a wiping part configured to receive the final air knife pressure from the air knife condition derivation unit and the final air knife gap from the air knife gap compensation unit and control the two air knives according to the final air knife pressure and the final air knife gap so that the target coating weight is achieved, 
 wherein the control period is a period for updating an air knife condition for the target coating weight, 
 wherein, when the control period is longer than the pressure response time period, the air knife pressure response compensation unit sets the gap compensation ratio to zero, 
 wherein, when the control period is equal to or shorter than a pressure response time period, the air knife gap compensation unit further configured to calculate the gap compensation ratio based on the air knife pressure variation amount for the control period and a value obtained by dividing the control period by the pressure response time period, multiply the gap compensation amount and the gap compensation ratio, and determine the final air knife gap by adding the value obtained by the multiplication to the first air knife gap, and 
 wherein the pressure response time period is the time period for which the current air knife pressure reaches the final air knife pressure. 
 
     
     
       2. The apparatus of  claim 1 , wherein:
 the gap compensation ratio follows an equation below,
   (gap compensation ratio)=1− Tc/ 2 Tp ,( Tp>=Tc )
 
   (gap compensation ratio)=0,( Tp<Tc ), 
 
 Tc is the control period, and Tp is the pressure response time period. 
 
     
     
       3. The apparatus of  claim 1 , wherein:
 the air knife gap compensation unit is further configured to add a quantization result of the value obtained by the multiplication to the first air knife gap. 
 
     
     
       4. The apparatus of  claim 1 , wherein:
 when an air knife pressure change rate within one control period is equal to or larger than a predetermined threshold rate, 
 the air knife gap compensation unit derives is configured to derive a third air knife gap for reaching the target coating weight at a current air knife pressure, and determines a final air knife gap according to a gap compensation amount that is a difference between the third air knife gap and the first air knife gap and the gap compensation ratio. 
 
     
     
       5. A method of controlling coating weight of a steel sheet by using two air knives, the method comprising:
 deriving, by a coating weight control apparatus, a first air knife gap and a final air knife pressure for target coating weight; 
 deriving, by the coating weight control apparatus, a second air knife gap for achieving the target coating weight at a current air knife pressure; 
 calculating, by the coating weight control apparatus, a gap compensation amount that is a difference between the second air knife gap and the first air knife gap; 
 calculating, by the coating weight control apparatus, a gap compensation ratio based on an air knife pressure variation amount for a control period; 
 determining, by the coating weight control apparatus, a final air knife gap based on the gap compensation amount and the gap compensation ratio; and 
 transmitting the final air knife pressure and the final air knife gap to a wiping part for controlling the two air knives so that the target coating weight is achieved, 
 wherein the control period is a period for updating an air knife condition for the target coating weight, 
 wherein the calculating of the gap compensation ratio includes: 
 when the control period is longer than the pressure response time period, setting the gap compensation ratio to zero, and 
 when the control period is equal to or shorter than a pressure response time period, calculating the gap compensation ratio based on the air knife pressure variation amount for the control period and a value obtained by dividing the control period by the pressure response time period, multiplying the gap compensation amount and the gap compensation ratio, and determining the final air knife gap by adding the value obtained by the multiplication to the first air knife gap, and 
 wherein the pressure response time period is the time period for which the current air knife pressure reaches the final air knife pressure. 
 
     
     
       6. The method of  claim 5 , wherein:
 the gap compensation ratio follows an equation below,
   (gap compensation ratio)=1− Tc/ 2 Tp ,( Tp>=Tc )
 
   (gap compensation ratio)=0,( Tp<Tc ), 
 
 Tc is the control cycle, and Tp is the pressure response time period. 
 
     
     
       7. The method of  claim 5 , wherein:
 the determining of the final air knife gap includes: adding a quantization result of the value obtained by the multiplication to the first air knife gap to determine the final air knife gap. 
 
     
     
       8. The method of  claim 5 , further comprising:
 when an air knife pressure change rate within one control cycle is equal to or larger than a predetermined threshold rate, 
 deriving a third air knife gap for reaching the target coating weight at a current air knife pressure; and 
 determining a final air knife gap according to a gap compensation amount that is a difference between the third air knife gap and the first air knife gap and the gap compensation ratio.